1. Field of the Invention
This invention relates to minimum shift keying communications systems.
2. Description of the Prior Art
With the ever increasing use of high density multi-channel radio communications, the electromagnetic energy spectrum has become overcrowded. Thus, in order to avoid inter-channel interference, strict control of communication ranges in such communication systems is desirable. Moreover, since most medium and high output power transmitters perform with higher efficiency when operating at or near saturation of their input versus output amplitude transfer function, the ability to utilize transmitters with non-linear amplitude transfer functions has become an important design criteria in such communication systems.
Frequency shift keying (FSK), a technique of communicating digital information using discrete frequencies to represent specified symbols, has properties which are desirable in controlling the bandwidth of the system and for use with non-linear transmitters. Binary FSK, for example, transmits a first (mark) frequency to represent a binary one and a second (space) frequency to represent a binary zero. The amplitude of the carrier, in the ideal case, is invariant and thus can pass through non-linear amplifiers with minimal degradation of signal quality. Further, proper choice of the difference between the mark and space frequencies and the time of switching between these frequencies can provide strict control and minimization of the radiated spectrum thereby minimizing the required channel bandwith and the spurious emission of power outside the channel bandwidth.
Minimum shift keying (MSK) is the special case of FSK wherein the difference between the mark and space frequency is at the minimum value still preserving orthogonality (zero cross-correlation) between the mark and space signals and thus providing for efficient detection of the data. Continuous phase is maintained at the transitions between the mark and space frequency signals to decrease the out-of-band signal power.
Such advantageous conditions are met by maintaining a phase-lock between the mark and space frequency signals and maintaining the separation between the mark frequency (f.sub.m) and the space frequency (f.sub.s) at one half of the bit rate (R) of the binary information to be communicated, that is, EQU f.sub.m - f.sub.s = R/2 (1)
it is assumed, for the purpose of discussion, that f.sub.m is greater than f.sub.s. Such a case, however, is not a requirement of or a limitation on the system.
For proper demodulation of such MSK signals, coherent replicas of the mark and space frequency signals must be generated in the receiver, i.e., the replicas must be equal in phase at the beginning of each bit. Prior art systems, such as that described in U.S. Pat. No. 3,743,775, to Hutchinson et al., issued July 3, 1973, generate such coherent reference signals by applying the received MSK signals to a square-law device, to remove phase dependence of the modulation signal on the particular data, and presenting the squared signal to two separate phaselocked loops, which respectively acquire and track mark frequency signals and space frequency signals. Thus coherent reference signals for the mark and space frequencies are generated.
However, where the information to be transmitted is such that there are long sequences of continuous ones or zeros, thereby respectively causing continuous transmission of mark frequency signals or space frequency signals, the phase-locked loop generating the reference signal for that frequency not being transmitted will drift. Thus, when a transition in the transmitted signal finally occurs, i.e. the absent frequency is again transmitted, the respective phase-locked loop will have to reaquire the frequency, during which time data will be lost. Moreover, derived reference signals for both the mark and space frequencies are required to derive a valid bit timing signal. Thus, bit timing errors will result during those periods when one or both of the phase-lcoked loops are not locked on their respective signals.
In conventional equipment this problem has been resolved by requiring a minimum bit transition density, i.e. a minimum number of transitions per word. However, a minimum bit transition density necessarily precludes communication of information which does not maintain the requisite minimum number of transitions.
The present invention overcomes the disadvantages and problems present in the prior art by providing coherent mark and space frequency references even in the absence of one of either mark or space frequency signals, and further eliminates any requirement for a minimum bit transition density. Moreover, the present invention includes provisions to compensate for fluctuation in the bit rate, doppler shift in the frequency of the transmitted signal and for frequency offset between receiver and transmitter.